I have an older Tigoat bivy, and it had a bag-style headnet that pulled from the zipper opening over the head of the bivy.
Having the full mesh hood is great for ventilation, view, and claustrophobia.
But if you are tall, or in any way pushing the size of the bivy, the full mesh hood has the least volume, which is where the window has thee advantage: it creates more volume.
Plus the full-mesh hood requires folding the fabric hood inside out, underneath the bivy, which means if you aren't using a ground cloth, the inside of the part of the bivy that goes over your face is in the dirt.
If I was getting the ptarmigan, I would go for the window, even though it sacrifices ventilation and view.